This invention relates to compositions that are suitable for use as conductive materials in microelectronic devices or semiconductor packages to provide electrically stable interconnections.
Conductive compositions are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. For example, conductive adhesives are used to bond integrated circuit chips to substrates (die attach adhesives) or circuit assemblies to printed wire boards (surface mount conductive adhesives).
Two conductors with dissimilar electrochemical potentials will form an electrochemical cell in the presence of water. The conductors act as cathode and anode, and environmental humidity provides the necessary aqueous medium to bridge the anode and cathode. The metal with the lower electrochemical potential acts as the anode resulting in the loss of electrons {Mxe2x88x92nexe2x86x92Mn+} and the corrosion of the metal. The metal with the higher electrochemical potential acts as the cathode {2H2O+O2+4exe2x86x924OHxe2x88x92}. Oxygen is involved in this mechanism but does not directly react with the anode metal. The metal ion Mn+will combine with OHxe2x88x92 and form a metal hydroxide that stabilizes by developing into a metal oxide, which over time forms on the anode surface. Metal oxides in general are non-conductive, the result being a decrease in conductivity of the metal circuitry.
The problem is less acute when the filler in the composition is the same metal as the contiguous circuitry or the substrate. Thus, a semiconductor package using a conductive composition, one comprising an epoxy resin and silver filler, for example, will not be as susceptible to electrochemical failure when a silver-filled composition is used on a silver substrate. However, if the composition is used on a nickel plated substrate, electrochemical corrosion will result under high humidity conditions.
These compositions, however, are vulnerable to environmental conditions, and high temperature and high humidity can cause the electrical resistance of the assembly fabricated with these compositions to increase substantially over time. The suspected mode of failure is electrochemical corrosion of the circuitry at the interface of the conductive filler in the composition with another contiguous metal surface, for example, a metal lead frame or other circuitry.
It would be an advantage, therefore, to provide conductive materials that form electrically stable assemblies for use in semiconductor packaging operations. It would also be advantageous to provide a conductive adhesive which would combine a corrosion inhibitor in a manner that would provide improved contact resistance when exposed to harsh environmental conditions.
This invention is a composition that comprises a polymeric resin, a conductive filler, one or more corrosion inhibitors, a curing agent/catalyst, an adhesion promoter and optionally other additives such as reactive or nonreactive diluents. The compositions exhibit improved electrical stability over those compositions that do not contain a corrosion inhibitor. The corrosion inhibitors will be present in an amount of no more than 10 weight percent (but not 0%); the resin will be present in an amount of 10 to 90 weight percent; the filler in an amount of about 1 to 90 weight percent; the curing agent will be present in a ratio of curing agent to resin in amount of no more than 1:1, the adhesion promoters in an amount of no more than 10 weight percent, for a total of 100 weight percent.
In a further embodiment, the composition includes N-Hydroxysuccinimide in combination with one or more corrosion inhibitors.